In the machining of conventional brass alloys which consist of 57-62% by weight copper, 1-4% by weight lead, balance zinc, and which are known as free-cutting brass alloys, the increasing use of automatic lathes imposes increasingly more stringent requirements as regards the machinability of such alloys and particularly as regards the edge life of the tool. The machinability is defined either by the edge life of the tool when used for a defined machining operation or by the increase of the cutting speed which results in a given edge life of the tool. In view of the urge toward higher efficiency, the edge life of the tool must be increased and all possiblities regarding the material must be utilized in order to develop free-cutting brass alloys which have the required strength and result in a minimum wear of the tool and in a maximum edge life of the tool.
For this purpose it is known to use a free-cutting brass alloy, which differs in composition from the conventional free-cutting brass alloy Cu45 Zn39 Pb3 only by a higher lead content of 4.25% by weight rather than 3.25% by weight. The increased lead content results in an improved machinability, giving a tool edge life which is 2.5 to 4 times longer at a given cutting speed or permitting an increase of the cutting speed by as much as 33% for a given edge life of the tool. That brass alloy has the significant disadvantage that the increased lead content adversely affects the mechanical properties of the alloy, particularly its strength and hardness, so that the alloy cannot be used in all cases to make highly stressed machine elements such as thin-walled elements having internal screw threads.
A lead-containing brass alloy has beed proposed which is subjected to a treatment to reduce the content of silicon dioxide and zinc sulfide inclusions and which contains a reagent metal which consists of at least one of the following components: Magnesium in an amount of 0.01-0.3% by weight, calcium in an amount of 0.001-0.05% by weight, sodium and/or potassium in an amount of 0.0001-0.0005% by weight, and mischmetal in an amount of about 0.14%. It has been found that the machinability of this brass alloy is not better than that of the conventional brass alloy Cu58 Zn39 Pb3.